Antenna



A. ALFORD May 16, 1950 ANTENNA Filed June 19, 1946 DIELECTRIC MA TERI/H.

Fig.2

9 ANDREW K L E SED WW pi ATTO RN EY Patented May 16, 1950 UN I'TED STATES PATENT OFFICE ANTENNA Andrew Alford, Cambridge, Mass.

Application June 19, 1946, Serial'No. 677,655

13 Claims. 1

This invention relates tonew and useful improvements in antennas andparticularly inmobile transmitting and receiving antennas for 'use onautomobiles, buses, trains, aircraft and the like.

'In'the art of radio transmission where at least one station is mobileitis the general practice *to employ" vertically polarized radiofrequency "waves, since they have good field strength near to theground.

A simple and widely used antenna for such waves is a vertical flexiblewhip antenna. This antenna has several serious shortcomings. In

order to function efficiently it must be near to a quarter wave lengthlong, a length at which it is resonant and has a narrowpass band. Ihepass band can be broadened by'increasingthe thickness of the radiatingelement but it then may cease to be a'whip antenna and may present toorigid and hazardous a projection from a moving "vehicle.

"wave whip'antennais of the order of ten per The band width of anordinary quarter cent (.10) and, even if compensating circuits are used,this-can only be increased to approximately twenty five per cent (.25)

If the length of a whip antenna is reduced below aquarter wave length,then its radiation resistance will'become materially lowered anditsinput'impedance will include capacitive reactance. The expedient oftuning out this re- -'actance with a coil will further decrease theefficiency ofthe antenna, and its already small band Width.

' Tilting a vertical whip antenna away from-its perpendicular position,so as to lower the height 'of'its tip, will diminish both its bandwidth'a'nd 1 radiation resistance.

At the relatively high frequency of 156 me. a quarter wave length Whipwould be all of- 16 inches long. A 16-inch protrusion from the top of avehicle as high as a-bus, or a truck, or a railroad car can be .verytroublesome. Such vehicles frequently are built nearly as high as theminimum head room tolerance legally permitted for bridges, tunnels,viaducts and terminals. Moreover, incertain-areas such obtrusions wouldmost likely encounter low' hanging branches of trees.

Foldable mounting of a whip antenna to avoid,

overhead obstacles'willmake continueduse of the associated radioequipment impossible. *Fur- -=thermore,- such a mounting, aswell as oneper- -mitting the :whipantenna to tiltwhen striking i an object,will'necessitate complicated and eX- i :pensive flexible feed line meansand will add to servicing requirements.

A' whip antenna attached to some part of a vehicle-J below the level ofthe roof, such as on a running board or bumper, will always be masked insome direction by the body of the vehicle.

Another possible objection to whip antennas is that they are conspicuousand easily-identified and may, therefore; constitute an obvious clue tothe presence of radio equipment in a vehicle and to. the type of vehicleit is, for exampie; that it is a special police car.

'The protrusion of a' whip antenna is especially :objectio'nable on anaircraft for the additional reasons that at ordinary high speeds it willproduce substantial drag'while at-very high speeds its drag will attainsuch an inordinately large .magnitude'thatit mayieither breal: off theantenna or damage the aircraft.

Among 'the'objects' of this invention are to provide a verticallypolarized transmitting or receivin antennawhich has aband width of 1.2and substantially'circular radiation patterns in horizontalplanes,"which= antenna is very compact and therefore will, wheninstalled on a vehicle, present no protrusion Whatever.

With this object in view I provide a hollow cconduc'ting'body, such as ametal cylinder, which is 'connected'with, and is'preferably carriedby, aconducting surface. The-body forms a transmissionline with said surfacewhich may be the roof of a'vehicle; a wing of an airplane, etc.

In accordance with one feature of the invention, the 'transmissionlineis formed by a gap between theinner side of an edge of a hollowconductingbody, such as a cylinder, which is completely within thevehicle, and the-perimeter of a portion of a fiat conducting surfaceconstituted by part of a". surface of the vehicle surrounded by saidledge, the'feeder being connected with the surface and the body: nearsaid gap.

According'toanother feature of the present invention, whenthe hollowconducting body is so proportioned that the area of an axial crosssection of theenclosedspace is of'the order of one-third'ofone-thousandth of awave length squared, then-the antenna satisfies theabovementioned objects.

' Reference is made to my copending applica tion' U. S. Serial No;669,758, filed May 15, 1946,

although parts of 'its'disclosure will be repeated for theconvenience'of-the reader and to facilitate the understanding of the presentinvention.

' 'Otherobjects, features and advantages of this invention will becomeapparent to'those familiar 3 with the art from the followingdescription, and the appended claims and drawings in which:

Fig. l is a top plane view of the radiating element of one embodiment ofthe antenna;

Fig. 2 is a transverse cross sectional view of the antenna; and,

Figure 3 shows a section of a modification of the invention in the samerelation as in Figure 2.

The antenna shown in the drawing consists of a cylindrical body elementI, which may be a metallic casting or spinning shaped like a pie plate.This element has a bottom disc 2, and a rim attached thereto, whichbears the numeral l2. The cylinder is provided with a tube 3 which A ismechanically attached to inside surface of bottom 2 at substantially itscenter. In practice tube 3 may be formed as an integral part of elementl, 2. Tube 3 may be of metal and as a result will be conductive.However, the inside may be non-conductive without preventing this devicefrom functioning. The outer surface of tube 3 is part of the inner areaof the hollow cylinder and should be conductive as there must be a lowimpedance axial connection between the inner sides of surfaces at theends of cylinder I, 2.

Tube 3 is of such length and is so arranged that a disc 4 carried by itsupper end is parallel with bottom disc 2 and lies in the plane of alarge plate 5 with which the rim of cylinder I, 2 is in contact and towhich it may be fastened. The disc and plate constitute in effect oneground plane for the cylinder. The rim of the cylinder l, 2 is evenlyseparated by gap 6 from the edge of disc 4. Gap 6 may be an air gap, butpreferably it is physically sealed by a dielectric substance 7. Gap 6 isshort-circu ited by a radial metal plate 8 within the cylinder extendingradially from tube 3 to rim I with its edges contacting discs 2 and 4.

The structure l--4 may be described as a hollow body or cylinder whichhas its ends closed by disc 2 and the disc 4 which is part of the groundplane, and has a gap 6 of about the same diameter as the cylinder whichreturns on itself.

The hollow cylinder, as well as the other elements, may be made of s01idsheet metal (or an equivalent such as solid sheet plastic including aconductive layer) and, accordingly, the inner and outer areas, willcomprise wall surfaces physically unbroken (except for gap 6 in caseswhere it is air insulated) However, this physical continuity is notnecessary, and the device will operate satisfactorily if made of wirescreen or of perforated sheet metal if the openings are not too largewith respect to one wave length corresponding to the highest operatingfrequency, e. g. not larger than /100 of a wave length.

The antenna is energized over a coaxial transmission line having anouter conductor 9 and inner conductor l9. Inner conductor I is connectedto the edge of the disc 4 and the outer conductor 9 is connected to theside wall of cylinder 2 and also to plate in which a hole II is formedfor its passage. These connections are made at points opposite eachother across gap 6 and this part of the gap is at a, place on the sideof the hollow cylinder which is diametrically opposite the position ofshort-circuiting bar 8.

When the antenna is energized the edges of gap 6 constitute a balancedtransmission line. The hollow body portion of this antenna (which may befurther described assurrounding and enclosing a toroidal space adjacentto gap 6) has distributed inductive reactance which is shunted acrossthe transmission line formed by gap 6. The magnitude of this distributedreactance depends primarily on the area of a transverse cross section,taken in the plane of plate 8, of the toroidal space on one side of tube3. This is the space between the inner surfaces of element I, 2, theouter surface of tube 3 and the inside surface of disc 4. If the insideradius of the hollow cylinder is designated as D, the outer radius ofthe tube 3 as d, and the distance between the inner surfaces of bottom 2and disc 4 as H, then this cross sectional area may be calculated byusing the formula A=(Dd):cH. Certain cylinders which have been made andtested and which have proper magnitudes of distributed reactance toperform in a preferred manner in accordance with this invention, havesuch dimensions that, when converted into wave lengths, the area derivedby solving the equation is of the order of one-third of one-thousandthof a wave length squared. To specific antennas were tested; in oneA=.00324 A and in the second it was .003.26 A

Short-circuiting plate 8 causes nearly complete reflection of incidentwaves traveling along gap 6 around both sides of the hollow cylindertoward it. The interference between incident waves and reflected wavesresults in a standing wave distribution of voltage across gap 6.

One minimum of voltage across the gap occurs at shorting plate 8; twomaxima of voltage occur at points on opposite sides of disc 4 each ofwhich is about midway between plate 8 and the feed point. Under certainconditions, two additional voltage minima, which ordinarily areundesirable, may occur on gap 6. They will be between plate 8 and thefeed point and are likely to be near to, and on each side of, the feedpoint. A condition which may cause this is that the gap is too long (thecircumference of disc 4) in proportion to the cross sectional area A andthat, therefore, the velocity of propagation along the gap is not greatenough for the gap length.

When the only voltage minima are at the shorting plate and the feedpoint, the voltage differences across gap 6, though they vary inmagnitude, are in the same phase, and currents moving to and from therespective edges of gap 6 at different positions along the gap movesymmetrically together. It ha been empirically ascertained in actualtests, and it can be further demonstrated in other ways, that thisinphase condition of the RF voltages across gap 6 and the circular shapeof the radiating portions will cause this element to act as ahorizontally omni-directional radiator of vertically polarized energy,assuming, of course, that disc 4 is parallel to the ground.

When undesired additional voltage minima occur on gap 6 between the feedpoint and plate 8, the phases of the RF voltages near those minima shiftrapidly. This alters the current distribution pattern and results inasymmetry of the horizontal radiation pattern. The hollow cylindershould preferably be so proportioned that the distances from plate 8 tothe first voltage minima (or to imaginary positions of the first minimaconstructively existing beyond the feed point) in each direction arounddisc 4 are equal to (or greater than) one-half of the circumference ofdisc 4.

The desired standing wave condition can be obtained even when each halfof gap 6 is greater than A/Z in length by sufficiently increasin theaeoaoes velocity. of propagation. along the gap. It can be increasedsubstantially beyond the. speed of lightunder certain conditions.controlled in part, within certain: limits, by the. cross sectional areadescribed above and by the capacity per unit length betweenthe edges ofthe gap.

A cross sectional area A=.00324 A together with a gap which is-a smallfraction of a wave length wide e. g. of the order of 045% wide, andwhich has one of its edges formed by a thin metal edge of disc l,results 'in-a velocity of propagation along the gap about 20- percentgreater than the velocity in free space. This permits-the" use of acylinder whose circumference is proportionally increased so thateachhalf circumference is in the neighborhood of twenty percent more than ahalf wave length. The physical enlargement of the hollow cylinder withrespect to the operating wave length, which is; effected at-the sametime thatundesired voltage minima are averted, will make its inputimpedance'low enough to match a 50-ohn1 coaxial line. Thus, in additionto being compact and vertically polarized (when properly positioned withrespect to the earth) the device requires no input transformer, makingit an ideal coupling between a 50,-ohm cable and the radiationresistance of space. One example of satisfactory proportioning of ,theseveral parts of the antenna may beoffered by suggesting the followingdimensions: D=,.485, d=.3027 H=.07l and the gap width, g=.0l42 Thesedimensions are, unless otherwise specified, expressed in terms of a wavelength correspondingto the middle fre quency of the operating band offrequencies. In one embodiment which was tested the midfrequency usedwas 1345 megacycles \=8.78") and in that embodiment D 4 d='2 H=%; and/8".

It should be borne in mind that the controlling geometriccharacteristicv for the hollow cylinder, if the gap be presumedto have afixed amount of distributed capacitance per unit length, is the crosssectional area described above. For example, d may be made larger withrespect to D, thus reducing the value of D-d, but by increasing H by aneasily determined amount-A will remain substantially the same and thestanding wave condition and input impedance will be substantiallyunchanged.

Moreover, the hollow, cylinder may be shaped so that its toroidalv crosssection on each side of the central tube is of other shapes thanrectangu lar. This cross sectionmay be circular, elliptical orirregularly shaped. Inthe samemanner, it is not essential that crosssections of the hollow cylinder, taken in planes perpendicular to theaxis of tube 3 shouldbe circular. The important factors are thatAbegenerally of the order oi one-third of a thousandth A and that the exactvalue of A can vary depending upon the capacitive characteristics of thegap. In this application, and in particular in the claims,.the wordcylinder is-usedinabroad'sense to indicate all hollow structures whichelectrically conform with the requirements of*theinvention, though theyare not. necessarily geometric cylinders" having circular transversecross sections.

The antenna produces substantially circular radiationpatterns inhorizontal planes, 1. e. planes perpendicular to the axis of tube:3,.and figureof-eght vertical patterns in planes passing through thataxis. The band width of the antenna is of, the order, of 1.2. Thebandwidth is commonly defined; as; a; ratio: of the highest irequency tothat of the; lowest'frequency trans mitted.

Obviously the antenna. described abovemay be employed .at frequenciesand for purposes other thanthose given above by way of exemplification.

The conductive ground plane 5 maybe a metal sheet attached to the roofof a vehicle,.or' to. the wing of an airplane, or may forman integralportion thereof. It does not necessarily. have to .be perfectly fiat..As an analogous example, it is known that a ground plane of finitephysical size, above a certain limit, will behave likean infinitereflective plane-and affect the radiation patterns of any radiatorassociated with it. .A physically infinite plane is unnecessary.Similarly, a physically substantially fiat plane will, at ordinary wavelengths, behave like a perfectly flat ground plane. Therefore, thecurved metal roof or other surface of a vehiclemay serve as groundplaneS.

It is obvious that ground plane 5 will-affect. the vertical distributionpattern of the radiator. However, this is not a disadvantage. If theantenna is close to the earth, e. g. on the roof of a bus, the areasmasked by the ground plane will be unimportant. Even if the antenna isnot close to the earth, 1. e. if it is in an airplane, the verticalmasking is not a disadvantage. For in an aircraft installation theradiating element could be installed on the underside of the surfaceacting as a ground plane. The important patterns, those inhcrizontalplanes, are not appreciably afiected.

The non-conductive space inside the hollow cylinder need not necessarilybe filled-with air. Instead, other dielectrics-may be used includingdielectrics which are gases, liquids and solids. However, substanceswhose dielectric constants differ from that of air will increase thedistributed capacity and reduce the velocity of propagation. Theproportions of the hollow cylinder should, therefore, be somewhataltered. Other changes and adaptations will readily suggest themselvesto those skilled in the art.

The element-described in the present application as radial plate 8 neednot in practice have this exact structure nor be of exactly zeroimpedance. The terms short circuit and shortcircuiting element as usedherein, both in the description and in the claims, are intended toinclude low impedance terminations, such as a short-circuiting wireacross gap 6 or a similarly connected coil having low inductance. Theshort circuit or short-circuiting element-must only be capable of,producing a substantial percentage of reflection.

Moreover, it is also within the scope of this invention that anotherfeed line 9', Figure 3, be connected at the point designated for thelocation of and in lieu of the shorting element. The desired standingwave condition on the gap or slot can be produced with this two feedpoint arrangement justv as well as by one in which there is a singlefeed point and a reflection point or low impedance point diametricallyopposite to it. It has been found that usually a double feed affects theoperating characteristics of the antenna by broadening the operatingfrequency band.

It is not essential that disc 3 should be mechanically separate anddistinguishable from dielectric substance lwhich, in the embodimentshown in the drawing, fills gapG. Instead, the whole openend of hollowcylinder I, 2 ma be closed with asingle disc of dielectric.,material,such as certain plastics, modified in its centerby a conductive elementto be the electrical equivalent of disc 4. For example, a conductiveplating, netting, coating, or paint could be placed on either surface ofthe single dielectric disc, over an area which would correspond to disc4 in size and location. In fact a wire netting could be molded into thesingle dielectric disc to take the place of disc 4. Accordingly, it isintended herein, and in particular in the claims, that the word disc, asused in this context, refers to an electrically effective disc andincludes alternate physical structures such as the ones described aboveWhat I claim is:

1. An antenna comprising a metal cylinder open at one and closed at theother end, said cylinder having a circular cross section of a diameterof about one-half of a wave length corresponding substantially to thecenter frequency of the operating band, a metal disc of smallerdiameter, a metal tube having one end fastened to the closed end of thecylinder and the other end to the disc and holding the disc in the oneplane with the edge of the open cylinder end, the circular gap betweenthe edge and the disc being a fraction of a wave length wide, a lead-inline having one conductor connected with the disc and the otherconductor with the cylinder at the gap, and a short circuit means acrossthe gap at a point diametrically opposite the points of connection ofthe lead-in line.

2. The antenna according to claim 1, and in which the cross sectionalarea of the toroidal space within the cylinder taken in a planecontaining the axis of said tube and on one side thereof issubstantially one-third of one-thousandth of a wave length squared.

3. An antenna comprising a metal cylinder open at one and closed at theother end, said cylinder having a circular cross section, a metal disc,a metal tube having one end fastened to the inside surface of the closedend of the cylinder and the other end to the disc and holding the discin one plane with the edge of the open cylinder end but spaced therefromto form a circular gap between the edge and the disc, a lead-in linehaving one conductor connected with the disc and the other conductorwith the cylinder near said gap, and a short circuit means across thegap at a point diametrically opposite the points of connection of thelead-in line.

4. An antenna comprising a metal cylinder .07), high, where Acorresponds substantially to the center frequency of the operatingfrequency band open at one and closed at the other end, said cylinderhaving a circular cross section of ABA diameter, a metal disc of .4566Adiameter, a metal tube having an outer diameter of 30x having one endfastened to the inner surface of the closed end of the cylinder and theother end to the disc and holding the disc in one plane with the edge ofthe open cylinder end, the circular gap between the edge and the discbeing .0142A wide, a concentric transmission line having its innerconductor connected with a point in the edge of the disc and its outerconductor connected with the cylinder near said gap at another pointopposite to said first-mentioned point, a short circuit means across thegap at a place diametrically opposite to the points of connection of theconcentric transmission line.

5. In an antenna, a hollow cylinder having closed ends and a gap formedin the closure of one end having the form of a closed loop, an internalconnection between the ends near the axis of the cylinder, means forfeeding the cylinder near the gap connected thereto near the 8 gap, anda short circuit means across the gap. the distances between said shortcircuit at the gap and the points of feed at the gap in either directionalong the gap being equal to or less than where A, corresponding to themean operating frequency is corrected for the velocity of propagationalong the gap producing minima on the gap only at the short circuit andnear to the connection of the feed means.

6. In an antenna, a hollow cylinder having closed ends and a gap formedin the closure of one end having the form of a closed loop, a connectionbetween the ends near the axis of the cylinder, means for feeding thecylinder near the gap, and a short circuit means across the gap, thedistances between said short circuit at the gap and the points of feedat the gap in either direction along the gap being equal to or less thanwhere A, corresponding to the mean operating frequency is corrected forthe velocity of propagation along the gap producing voltage maxima onthe gap between the short circuit and the feed point which aresubstantially midway between the short circuit and the feed point.

7. In an antenna, a hollow cylinder having closed ends and a gap formedin the closure of one end having the form of a closed loop, a connectionbetween the ends near the axis of the cylinder, means for feeding thecylinder across the gap, and a short circuit means across the gap thedistance along the gap from the short circuit to the feed point ineither direction being less than the distance necessary to produce avoltage minimum on either side of the feed point.

8. In an antenna, a hollow conducting cylinder, a closure for one end ofthe cylinder, a closure for the other end of the cylinder extendingbeyond the side wall and having a gap which is substantially concentricwith the cylinder and within the circumference of the cylindrical wall,means providing an internal connection forming a continuous wall betweenthe closures near the axis of the cylinder, and means for feeding thecylinder across the gap.

9. In an antenna, a hollow conducting body having an edge, meansproviding a flat conducting surface extending in the same plane as saidedge and connected thereto, and having a nonconductive gap formed insaid surface adjacent to and within the boundary formed by the edgeforming a transmission line which has the form of a circle, a feederconnected with the body and the surface at the transmission line, andlow impedance radio frequency energy reflecting means connected acrossthe line diametrically opposite the feed point.

10. In an antenna, a hollow conducting body having an edge, meansproviding a flat conducting surface extending in the same plane as saidedge and connected thereto, and having a nonconductive gap formed insaid surface adjacent to and within the boundary formed by the edgeforming a transmission line which has the form of a circle, a feederconnected with the body and the surface at the transmission line, and asecond feeder similarly connected across the line diametrically oppositethe feed point.

11. An antenna having means providing a flat conductive surface having anon-conductive gap formed therein, having the form of a loop, a hollowconductive body having an outer wall con- REFERENCES CITED ductivelyjoined to said means on the outer side The following references are ofrecord in the of the gap and an inner wall having two ends, file of thispatent: one end conductively joined to the outer wall on its inner sideand the other end conductively 5 UNITED STATES PATENTS joined to thesurface on the other side of the gap Number Name Date forming a chamber,with the edges of the mate- 2,206,923 Southworth July 9, 1940 rialadjacent the gap forming a transmission line, 2,414,266 Lindenblad Jan.14, 1947 and a feeder connected with the transmission line 2,488,419Lindenblad Nov. 15, 1949 at points on opposite sides of said gap. 10

12. The antenna according to claim 11 in which FOREIGN PATENTS thetransmission line forms a circle. Number Country Date 493,695 GreatBritain Oct. 13, 1938 13. The antenna according to claim 11 in which thetransmission line forms a circle and in which short circuit means areconnected across the 15 transmission line substantially opposite theconnection of the feeder.

ANDREW ALFORD.

